1. Field of the Invention
This invention relates to driver circuits, and more particularly, to a method and system for compensating the slew rate of a driver circuit.
2. Description of the Prior Art
The invention of Integrated circuits (ICs) had a significant impact on the field of electrical engineering. Incorporating all circuits into a single chip has enabled miniscule devices such as the modern laptop computer and cell phones to be developed. Many ICs comprise off-chip driver circuits (OCDs) utilized for driving logic levels related to supply voltages for the IC off-chip. The OCD also functions as a protection against high voltages. A problem associated with off-chip drivers, however, is that the output slew rate has a wide variation caused by tolerances in process, voltage, temperature and output load.
In order to combat this problem, circuits for controlling an output slew rate have been developed. These systems aim to control the slew rate by enabling or disabling pre-driver devices. For example, certain control pins can be enabled or disabled so the gates of the output stage become correspondingly faster or slower. For a faster transition all pre-drivers must be turned on, whereas for a slower transition only one pre-driver needs to be turned on. Enable signals for pre-drivers are utilized to enable the pre-drivers. Calibration must be manual, however, as it is not known how many pre-drivers initially need to be enabled. This method is costly to implement, and a user will have to use a trial and error approach when determining certain parameters to configure the pre-driver circuit.
In order to provide a more automated approach to the slew rate control, other systems were developed. Please refer to FIG. 1. FIG. 1 is a diagram of a system 100 for controlling slew rate variation of an off-chip driver 160. As can be seen from the diagram, the system 100 comprises a delay locked loop (DLL) 110 consisting of a series of delay elements 112, a frequency divider (not shown) and a phase comparator (not shown). The output of the DLL 110 is coupled to a first XOR gate 120, which generates an ideal slew pulse. A rising (or falling) edge of the off-chip driver circuit 160 is input to a first comparator 152 and a second comparator 154 where the first comparator 152 switches at a high point and the second comparator 154 switches at a low point. The outputs of the first and second comparators 152, 154 are coupled to a second XOR gate 140, which generates a pulse that is proportional to an actual rise (or fall) time of the off-chip driver 160. The pulse from the first XOR gate 120 and the pulse from the second XOR gate 140 are then fed into a phase comparator 130 which generates a signal that determines whether or not to increase the strength of the pre-driver 165 in the off-chip driver 160.
Although this method is effective, the system 100 requires many elements, and the DLL 110 must also be set at the start of a slew rate compensation operation.